Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Other applications, requiring very precise, non-contact liquid pattern deposition, may be served by drop emitters having similar characteristics to very high resolution ink jet printheads. By very high resolution liquid layer patterns, it is meant, herein, patterns formed of pattern cells (pixels) having spatial densities of at least 300 per inch in two dimensions. It is further meant that the liquid may be incrementally metered within a pattern cell in multiple subunits to produce a “grey scale” effect, using smallest unit drop volumes of less than 10 pL.
Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet or continuous ink jet. The first technology, “drop-on-demand” ink jet printing, provides ink droplets that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). Many commonly practiced drop-on-demand technologies use thermal actuation to eject ink droplets from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink droplet. Other well known drop-on-demand droplet ejection mechanisms include piezoelectric actuators.
Drop-on-demand drop emitter systems are limited in the drop repetition frequency that is sustainable from an individual nozzle. In order to produce consistent drop volumes and to counteract front face flooding, the ink supply is typically held at a slightly negative pressure. The time required to re-fill the drop generation chambers and passages, including some settling time, limits the drop repetition frequency. Drop repetition frequencies ranging up to ˜50 KHz may be possible for drops having volumes of 10 picoLiters (pL) or less. However, a drop frequency maximum of 50 KHz limits the usefulness of drop-on-demand emitters for high quality patterned layer deposition to process speeds below ˜0.5 m/sec.
The second ink jet technology, commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source that produces a continuous stream of ink droplets from a nozzle. The stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break-off length, from the nozzle. Since the source of pressure is remote from the nozzle (typically a pump is used to feed pressurized ink to the printhead), the space occupied by the nozzles is very small. CIJ drop generators do not have a “refill” limitation since the drop formation process occurs after ejection from the nozzle, and thus can operate at frequencies approaching a megahertz. In light of these characteristics, it is surprising that CIJ drop generators have not been employed in high density arrays for very high speed, very high quality deposition of materials. However, despite the need for apparatus to effect such deposition, for example apparatus to deposit high resolution patterns of electronic materials, no high density arrays have been reported or commercialized.
CIJ drop generators rely on the physics of an unconstrained fluid jet, first analyzed in two dimensions by F. R. S. (Lord) Rayleigh, “Instability of jets,” Proc. London Math. Soc. 10 (4), published in 1878. Lord Rayleigh's analysis showed that liquid under pressure, P, will stream out of a hole, the nozzle, forming a jet of diameter, Dj, moving at a velocity, vj. The jet diameter, Dj, is approximately equal to the effective nozzle diameter, Dn, and the jet velocity is proportional to the square root of the reservoir pressure, P. Rayleigh's analysis showed that the jet will naturally break up into drops of varying sizes based on surface waves that have wavelengths, λ, longer than πDj, i.e. λ≧πDj. Rayleigh's analysis also showed that particular surface wavelengths would become dominate if initiated at a large enough magnitude, thereby “synchronizing” the jet to produce mono-sized drops. Individual CIJ drop generators or low density arrays of CIJ drop generators may be configured to produce the 100's of 1000's of small (>10 pL) drops per second, which is one of the requirements needed for high quality patterned layered deposition process speeds above 0.5 m/sec.
However, large arrays of CIJ jets having jets spaced more closely than 300 jets per inch, meeting the requirements desired for high quality patterned deposition of materials, have been difficult to fabricate using conventional nozzle fabrication methods such as nickel electroforming and drop generator assembly of multiple layers and piece parts. In addition, commercially practiced CIJ printheads use a piezoelectric device, acoustically coupled to the printhead, to initiate a dominant surface wave on the jet leading to “Rayleigh” break-up into streams of mono-sized drops. It is quite difficult to produce uniform acoustic stimulation for long arrays of closely spaced jets. Further, conventional CIJ nozzle fabrication methods have not been successful producing long arrays of nozzles having diameters less than 15 microns, as is needed to form drops of less than 10 pL.
Because of the difficulties of traditional CIJ fabrication techniques and acoustic stimulation, even though the continuous drop emission process is capable of high drop repetition frequencies, practical systems comprising large arrays of CIJ nozzles that can produce a very high resolution patterned layer at process speeds above 0.5 m/sec have not been commercially realized, despite the need for such arrays for use in the printing of images and for patterning materials, such as thin-film electronic materials, a market widely acknowledged to be growing and potentially lucrative. An alternate jet perturbation concept that overcomes the drawbacks of acoustic stimulation was disclosed for a single jet CIJ system in U.S. Pat. No. 3,878,519 issued Apr. 15, 1975 to J. Eaton (Eaton hereinafter). Eaton discloses the thermal stimulation of a jet fluid filament by means of localized light energy or by means of a resistive heater located at the nozzle, the point of formation of the fluid jet. Eaton explains that the fluid properties, especially the surface tension, of a heated portion of a jet may be sufficiently changed with respect to an unheated portion to cause a localized change in the diameter of the jet, thereby launching a dominant surface wave if applied at an appropriate frequency.
Eaton teaches his invention using calculational examples and parameters relevant to a state-of-the-art ink jet printing application circa the early 1970's, i.e. a drop frequency of 100 KHz and a nozzle diameter of ˜25 microns leading to drop volumes of ˜60 pL. Eaton does not teach or disclose how to configure or operate a thermally-stimulated CIJ printhead that would be needed to print drops an order of magnitude smaller and at substantially higher drop frequencies.
U.S. Pat. No. 4,638,328 issued Jan. 20, 1987 to Drake, et al. (Drake hereinafter) discloses a thermally-stimulated multi-jet CIJ drop generator fabricated in an analogous fashion to a thermal ink jet device. That is, Drake discloses the operation of a traditional thermal ink jet (TIJ) edgeshooter or roofshooter device in CIJ mode by supplying high pressure ink and applying energy pulses to the heaters sufficient to cause synchronized break-off but not so as to generate vapor bubbles. The inventions claimed and taught by Drake are specific to CIJ devices fabricated using two substrates that are bonded together, one substrate being planar and having heater electrodes and the other having topographical features that form individual ink channels and a common ink supply manifold. Drake does not disclose a high resolution, very high speed CIJ configuration
Thermally stimulated CIJ devices may be fabricated using emerging microelectromechanical (MEMS) fabrication methods and materials. By applying microelectronic fabrication process accuracies to the construction of a thermally stimulated CIJ drop emitter, the inventors of the present inventions have realized that a liquid pattern deposition apparatus may be provided having heretofore unknown resolution and process speed capability. The physical parameters relating to continuous stream drop formation are constrained within certain boundaries to ensure the capability of providing a desired combination of pattern resolution, grey scale, drop volume uniformity, minimization of mist and spatter, and process speed. Such an apparatus has application for very high speed, photographic quality printing as well as for manufacturing applications requiring the non-contact deposition of high precision patterned liquid layers. The ability of MEMS fabrication methods to provide very high speed, high quality deposition of materials has heretofore been unrecognized, because an analysis of the many device and device fabrication parameters and of the design rules for the manufacture of such devices has not been undertaken. Although experimental devices have been built and disclosed that satisfy some of the requirements of high speed, high quality materials deposition, unguided experimental exploration of the many design and operational parameters of thermally stimulated CIJ printheads has failed to provide functional arrays of CIJ nozzles capable of high speed, high quality materials deposition. Such an analysis must include recognition of the implications of MEMS fabrication technologies as applied to thermally the stimulated inkjet devices.